Speed reducing planetary gear systems have been used for aircraft propulsion systems in either the dual output shaft mode, in which power is extracted from both the planet gear carrier and the ring gear, rotating in opposite directions, or the single output mode, in which either the ring gear or the planet carrier is fixed to the engine frame. In the class of high by-pass ratio aircraft gas turbine engines, the low pressure turbine shaft, rotating at high speed, typically directly drives the sun gear of a planetary gear system. This gear system includes a planet gear carrier supporting a plurality of planet gears and transferring power and load from the sun gear through the planet gears to one or both output shafts. High speeds and loads are imposed on this gear system, increasing with aircraft propulsion power requirements.
FIG. 1 illustrates a prior planetary gear system. The prior art planetary gear assembly 10 includes a ring gear 12 which is rigidly supported on support 14. Sun gear 16 is centrally located within the ring gear. Five planet gears 18 are arranged to mesh with the sun gear 16 and ring gear 12.
Each planet gear 18 has an outer gear section 20 with a bearing sleeve 22 fitted inside. The planet gear is supported on pin 24 which in turn is supported by the planet gear carrier 26.
Sun gear 16 is rotated in the direction 28 applying force 30 against the planet gear 18. This causes the planet gear to rotate in the direction of arrow 32 because of the force 34 imposed by the ring gear 12. The planet gears 18 and the planet gear carrier 26 move in the direction 36.
Centrifugal force imposes a G-force 38 on the planet gear. The combination of these forces results in a resultant force 40 transferred from the planet gear bearing 22 through a lubricating oil film to the pin 24, and thence to the carrier 26.
Previous gear systems employed steel-backed cast leaded bronze sleeves (item 22 in FIG. 1) installed in the planet gears with a very tight shrink fit. The sleeve 22 rotates around the pin 24. The cast leaded bronze bore of the sleeve acts as the journal bearing surface. Deflection of the pin, the carrier or the planet gear leads to distortion of the bearing surface and to localized variations in the bearing clearance. A balance is required between a minimum clearance and excessive clearance in order for the bearing to function properly. Too small a clearance results in excessive lubricant film temperature, and too large a clearance will result in inadequate film thickness.
It is known that flexibility of principal components of the gear system is necessary not only as a consequence of design and material selection for weight reduction, but also to contribute to load equalization between the various planet gears. However, flexibility of the planet gear must be carefully analyzed, since gear rim deflection not only modifies bearing clearance, but also disturbs the shrink fit between the bearing sleeve and the planet gear rim, potentially leading to wear at the shrink fit interface and failure due to mis-location of the bearing sleeve.
The load on the planet bearing which must be transferred through the pin to the carrier includes the sun gear force plus the ring gear force and the G-field centrifugal force due to planet gear mass. For a specified gear system outer diameter, a larger pin would stiffen the pin surface and lead to lower planet weight and also lower G-forces, but in turn increase planet distortion. It would also stiffen the carrier assembly and diminish load sharing capability between the various planet gears. A smaller diameter pin would facilitate load sharing but increases planet G-forces bending of the pin.
The prior art bearing sleeves have to be fabricated separately, assembled into the planet gear, retained mechanically with suitable features, and machine finished. These sleeves also add weight and cost to the prior art planetary gear systems. Furthermore, the separate steel backed bearing sleeve found in previous gear systems requires increasingly tighter shrink fit with the planet gear as gear distortion increases with increasing mesh loads, G-Forces and/or gear flexibility. The present invention overcomes these disadvantages by providing an integral one-piece construction of a planet gear having a bearing coating directly applied to the inner bore surface of the gear.
The following reference and those referred to hereinafter, which are each hereby incorporated by reference, disclose the state of the art.
U.S. Pat. No. 5,102,379 to Pagluica et al. discloses a planetary gear system which includes a plurality of planet gears, each having a journal bearing and a pin (see abstract). In Pagluica et al., the journal bearing must be fabricated separately, inserted into the planet gear and machine finished. This adds additional weight and costs, while reducing reliability.